With the advent and rapid growth of microprocessor technology, the present trend in the development of electronic systems and subsystems is an increasing use of adaptive techniques to optimize system needs and performance. Typically, each adaptive system requires one or more components such as complex weight devices for magnitude and phase adjustment of signals upon which the system operates. For example, a large-sized adaptive phased-array antenna requires several thousand complex weights in order to steer the antenna beam (or beams). For a system having such a large number of signal adjustment components, it goes without saying that the need for savings in energy, size and cost of the components has become a paramount concern, particularly in microwave communication systems where a variety of diverse elements are required to meet the functional demands of the system.
Conventional devices such as passive power combiner/dividers that have been employed in components such as phase shifters, complex weighting devices and switches, have been fabricated in a variety of configurations such as rat-race hybrids, edge couplers, interdigital couplers, branch line couplers and Wilkinson splitters. Disadvantageously, these passive types of components suffer from lack of gain, insertion loss and dividing loss, absence of reverse isolation and adjustability, and large physical size. As one example, PIN diodes, that have been conventionally used in complex weighting devices, are extremely lossy and suffer from high thermal noise, slow settling time, low dynamic range and poor tracking capability.
Presently, the trend in component fabrication is the use of monolithic microwave integrated circuits (MMIC), which offer low cost, miniaturization and high reliability. In such circuits, semiconductor material such as GaAs, InP and other III-V components are ideal substrates for making it possible to integrate a large number of components such as signal sources, modulators, mixers, amplifiers, multipliers, limiters, switches, etc. together to obtain these objectives. Unfortunately, the above-referenced conventional components do not lend themselves to monolithic integration with MMICs because of both material and diffusion process incompatibilities, in addition to their size and performance drawbacks.